The present invention relates to a sheet feed apparatus for a lithographic printing machine or the like. More specifically, the present invention relates to an apparatus comprising a drum formed with vacuum openings for suckingly holding a sheet thereto for printing or the like.
In this type of apparatus, a drum is formed with openings such as slots and a sheet is fed thereto and suckingly held to the drum by vacuum applied to the openings. With the sheet held thusly tight to the drum, an image is printed on the sheet by a lithographic or other printing process, after which the sheet is fed away from the drum.
A problem which has remained unsolved in this type of apparatus is that sheet feed failures occasionally occur which must be detected and cleared before further printing operations can be performed. Conventional means for sensing for sheet feed failures utilize microswitches, photosensors and the like to detect the feeding of a sheet to a particular position. Failure of the sheet to be sensed means that a sheet feed failure has occurred. Such sensors are costly and involve complicated and expensive auxiliary circuitry.
Another problem is that the level of vacuum must be adjusted to correspond to the thickness of the sheet being fed. The obvious solution is to provide a regulator valve which is set by the operator after selecting the thickness of the sheets. Such a regulator valve is highly subject to missetting or being ignored.
Yet another problem is that different levels of vacuum must be provided for feeding the sheet to the drum and feeding the sheet away from the drum. If the vacuum is set to the proper value for feeding the sheet to the drum and unchanged while feeding the sheet away from the drum, the level of vacuum will be too low and the sheet will not be fed away from the drum. Conversely, if the vacuum is set to a high value for feeding sheets away from the drum and employed for feeding sheets to the drum, there is a good possibility that two sheets will be fed to the drum.
Such changeover of levels of vacuum may be accomplished by means of two or more vacuum sources and selector valve means. However, the cost of such an arrangement is unreasonably high and changeover generally requires more than 12 seconds before stable vacuum is achieved.